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estimate the possible maximum and minimum height of that corner. This height range can be transformed into parallax 
range along the epipolar line. 
If there are more than one point in this range, the one which is nearest to a horizontal line in the object space will be 
chosen as the correct one and displayed on the screen for confirmation. The finding of the horizontal line will be 
explained in the next section. If no corresponding point can be found in the Point Database, that corner will be declared 
as a failure one. This can happen very often for the urban area due to occlusions 
After getting the 3-D coordinates of at least three corners, the roof patch can be inferred from the object knowledge of 
the roof patch model. That is the form of roof patch in the X-Y plane should be parallelism when only 3 corners are 
available. The height of the missing corner can be deduced according to the semantic information verified by the 
operator at the beginning of processing. 
After the reconstruction is finished, there is still the possibility for the operator to modify it interactively. The operator 
could modify the roof corners by choosing the optimal corner 
equivalent from one of the three different types of points in the 
Point Database or he can simply choose to modify the corner 
  
    
  
Corners of Roof Patch 
  
Yes Interactive Operation for Modification manually. 
1 Searching for the Structure Point From the above descriptions we can see that the advantage of the 
2.Searching for the Junction Point interactive approach in our system is that the operator can control 
3.Searching the 3-D Point | and modify the result at any time and in any processing step 
4.Manual Correction with Mouse Device 
  
  
  
using his high capability of interpretation. Yet the operator needs 
not to “plot” any thing very accurately. 
In case that the modified result is still not acceptable to the 
operator, there is the last possibility for the operator to 
reconstruct the roof patch manually, i.e. to “plot” individual 
corner points by dragging mouse cursor. The diagram of 
interactive approach for the modification of the corners is 
—No— —( Output ) illustrated in Fig.4. The workflow in Fig.4 is not only used in the 
final stage but also used in all stages through out the whole 
Fig.4. Diagram of Interactive Modification for Roof Corners reconstruction process. 
  
Next Corner Yes Verification 
  
  
  
3 POINT DATABASE AND POSITION OF CORNERS IN IMAGE SPACE 
The Point Database consists of three different types of points. They are the extracted junction points, the intersection 
points of linear segments (structure points) and the 3-D points. Among them only 3-D points have known 3-D 
coordinates in object space. A 3-D point is obtained by matching of the extracted 2-D lines in the early stage of finding 
*meaningful" lines. Their location in the left and the right images are obtained by back projection of the object point 
into the image space. There are two Point Databases in our system: one for left image and the other for the right image. 
There are two major motivations for construction the Point Database. One motivation is that the roof corner usually 
appears as point feature in image. It will be easier for us to search and locate the corners if there is a database of points. 
The other reason is that not only point type but also other useful information can be transferred and embodied in this 
database. For example, the relation of a point to adjacent lines can be included in this database. If some attributes of the 
points can be obtained and stored in the database, e.g. structure points from the intersection of L type lines or from the T 
type lines etc, to the search will be more effectively. 
Our interactive system will employ this Point Database in 
. three processing stages. The first application of this 
. database is to locate the lacking corners of quadrangular 
roof patch in single images. After the operator drags the 
cursor to point to those corners, the system will 
automatically try to lock a point of the type of the structure 
point or the type of the extracted junction point within 
certain range from the current cursor position. If more than 
Locked Point on the Line Locked Points nearby the Line one points are available, the system will choose the one 
which is nearest to the interactively selected 2-D line on the 
Fig.5. The Illustration of Positioned Location condition that the corner must locate along the direction of 
Left : Locked Point is the Positioned Position that line. If the point is just on this selected line, then it is 
locked by the system. If no point is exactly on this 2-D 
selected line, then the system will find the one with the 
the Line is the positioned Location shortest distance to this 2-D line as the locked point. Fig. 5 
Right: Location with the shortest distance to 
  
International Archives of Photogrammetry and Remote Sensing. Vol. XXXIII, Part B3. Amsterdam 2000. 187